Splice Types and When They Are Required

Beam splices are required when:

  1. Transportation limits: Sections longer than 12-15 m cannot be transported economically on Australian roads. Maximum truck length is typically 12.5 m for semi-trailers on standard routes (National Heavy Vehicle Regulator Class 1 permits may allow longer).
  2. Erection constraints: Crane capacity or site access may limit the weight of individual lifts. A 610UB125 beam of 15 m length weighs approximately 1.9 tonnes — within crane capacity, but a 20 m section would require a larger crane or a mid-span splice.
  3. Design optimisation: Where the bending moment diagram varies significantly along the beam, a splice allows the use of a lighter section where moments are lower, reducing overall steel tonnage.

AS 4100 Clause 9.5 requires that splices develop the full capacity of the spliced members, or the forces at the splice location from the governing load combination, whichever governs. This is a strength-level requirement — the splice must not be the weak link in the load path.

Bolted Flange Plate Splice — Moment Transfer

The moment at the splice is transferred through flange cover plates that develop the flange force through bolt groups.

Design Steps per AS 4100 Clause 9.5.2

Step 1 — Determine the flange force:

Tf = M* / (d - tf)

Where M* is the factored moment at the splice, d is the section depth, and tf is the flange thickness. The lever arm (d - tf) is the distance between flange centroids. For sections with unequal flanges, use the weighted centroid distance.

Step 2 — Size the flange cover plate:

The cover plate cross-sectional area must equal or exceed the flange area: Ap >= Af = bf x tf

The plate width is typically equal to the flange width (for visual uniformity), or slightly narrower to allow for access. The thickness is then determined as: tp_required >= Af / bp_selected

Step 3 — Design the bolt group at each flange:

Each side of the splice requires a bolt group to transfer the flange force from the beam flange into the cover plate. The bolt group is loaded in shear (single shear per bolt if the bolt passes through flange and cover plate; double shear if there are two cover plates, one each side of the flange).

Number of bolts per side per flange: n >= Tf / (phi x Vfn)

Where phi x Vfn is the design shear capacity per bolt per AS 4100 Clause 9.3.2.3. For Grade 8.8 M20 bolts with threads in the shear plane: phi_Vfn = 0.80 x 0.62 x 830 x 314 / 1000 = 129.3 kN per bolt. With threads excluded: 0.80 x 0.80 x 830 x 314 / 1000 = 166.8 kN per bolt.

Step 4 — Check bolt spacing and edge distances:

Per AS 4100 Clause 14.3.5:

Step 5 — Check cover plate net section at bolt holes:

The cover plate must be checked at the net section through the bolt holes for the row of bolts closest to the splice centreline. The net area is: An = tp x (bp - nh x dh)

Where nh is the number of holes in the critical row and dh is the hole diameter (typically bolt diameter + 2 mm for standard holes per AS 4100 Table 14.3.2).

Tension capacity of net section: phi_Nt = 0.90 x fu x An x kt

Where fu is the plate tensile strength (440 MPa for Grade 300 plate) and kt = 0.85 for bolted connections.

Web Plate Splice — Shear Transfer

The shear at the splice is transferred through web cover plates (one each side, or a single thicker plate). The web plate splice must develop the full design shear V*.

Design shear per bolt: Bolts through the web splice plates are loaded in double shear (one shear plane at web-to-plate interface on each side).

For M20 Grade 8.8 bolts in double shear with threads excluded: phi_Vfn = 0.80 x 0.80 x 830 x 314 x 2 / 1000 = 333.6 kN per bolt (two shear planes)

Number of bolts: n >= V* / (phi x Vfn_per_bolt)

Web cover plate thickness: Per AS 4100 Clause 9.5.3, the web cover plate thickness should be at least tw (the beam web thickness) for each plate. When two plates are used (one each side), each must be at least tw/2 in thickness, but a minimum practical thickness of 6 mm applies.

Bolt bearing on web plate: The bearing capacity per bolt on the web plate must also be checked: phi_Vb = phi x 3.2 x df x tp x fu (for edge bolts, where fu is the plate ultimate strength) phi_Vb = phi x ae x tp x fu (where ae = e - df/2, for bolts away from edges)

Worked Example — 610UB125 Beam Splice

Problem: A 610UB125 beam (Grade 300) requires a field splice at mid-span due to transport length constraints. Design a bolted splice for M* = 1,250 kN.m and V* = 350 kN at the splice location.

Section properties — 610UB125: d = 612 mm, bf = 229 mm, tf = 19.6 mm, tw = 11.9 mm Af = 229 x 19.6 = 4,488 mm^2 fy = 280 MPa (Grade 300, 12 < tf <= 20 mm)

Step 1 — Flange force: Lever arm = d - tf = 612 - 19.6 = 592.4 mm Tf = M* / 592.4 = 1,250,000,000 / 592.4 = 2,110 kN per flange

Step 2 — Flange cover plates: Required plate area = 4,488 mm^2 Use 230 x 20 mm plate: Ap = 230 x 20 = 4,600 mm^2 > 4,488 mm^2. OK. Plate steel: Grade 300 (fy = 300 MPa for t <= 20 mm).

Step 3 — Bolts per flange (8.8/S category, M24 bolts, threads excluded from shear plane): phi_Vfn per bolt = 0.80 x 0.80 x 830 x 353 / 1000 = 187.5 kN (single shear) Number of bolts required per side = 2,110 / 187.5 = 11.3 → use 12 bolts per flange per side. Arrange in 2 rows of 6 bolts. Pitch = 70 mm (> 2.5 x 24 = 60 mm).

Step 4 — Web splice: V* = 350 kN. Use 2 x 8 mm web cover plates (Grade 300, each side). M20 bolts in double shear: phi_Vfn = 260.2 kN per bolt. Bolts required = 350 / 260.2 = 1.34 → use 4 bolts per side (2 columns x 2 rows).

Step 5 — Bearing check on 20 mm cover plate (M24 bolt, edge distance 40 mm): phi_Vb = 0.90 x 3.2 x 24 x 20 x 440 / 1000 = 608 kN per bolt > 187.5 kN shear. OK.

Step 6 — Net section check for cover plate (12 bolts, 2 rows of 6, 2 holes per critical row): Net width = 230 - 2 x 26 = 178 mm (hole diameter = 24 + 2 = 26 mm) An = 20 x 178 = 3,560 mm^2 phi_Nt = 0.90 x 440 x 3,560 x 0.85 / 1000 = 1,198 kN Flange force per bolt row = 2,110 / 6 bolts per row x 2 bolts in critical row = 703 kN < 1,198 kN. OK.

Final specification: 230 x 20 mm flange cover plates (Grade 300), 12-M24 Grade 8.8 bolts per flange per side (2 rows x 6), 8 mm web cover plates both sides, 4-M20 Grade 8.8 bolts per side for shear. Splice conforms to AS 4100:2020 Clause 9.5.

Construction Considerations

Bolt tightening: All structural bolts in beam splices should be tightened to the snug-tight condition as a minimum. For slip-critical connections (fatigue loading, reversal, or where slip would affect structural performance), specify pretensioned bolts per AS 4100 Clause 15.2.5. Pretensioning methods: torque control, turn-of-nut, or direct tension indicator washers.

Splice location: Position splices at approximately one-quarter span where moments are lower. Avoid splices at points of maximum moment (mid-span for simply supported beams, supports for cantilevers) unless unavoidable. For continuous beams, position splices near points of contraflexure.

Alignment: Provide a backing bar or erection cleat to align the beam sections before bolting. The cleat can be tack-welded to one beam section in the shop and field-bolted to the other section, then removed after splice bolting if desired.

Surface preparation: Contact surfaces for bolted splices should be clean and free of paint, oil, and mill scale within the bolt group area. For slip-critical splices, Class 2.5 surface preparation (grit blast to Sa 2.5 per AS 1627.4) may be required.

Frequently Asked Questions

When should a beam splice be welded rather than bolted? Welded splices are used when the splice is shop-fabricated (no field access constraints) and when a full-strength moment connection is required without bolt slip. Field welding is generally avoided in Australian practice due to access, weather, inspection, and quality control difficulties. Bolted splices are strongly preferred for field connections because they are faster to erect, require less skilled labour, and are easier to inspect. AS 4100 does not prohibit field welding, but the ASI recommends bolted field splices wherever practical.

What is the difference between a bearing splice and a slip-critical splice? A bearing splice relies on bolts bearing against the plate material to transfer load — bolts are in shear and plates in bearing. Some slip occurs as holes engage. A slip-critical splice uses pretensioned bolts to develop friction between the plies, transferring load without slip at the serviceability limit state. Slip-critical splices are required for: fatigue loading, connections subject to stress reversal, and where slip would affect the load distribution in the structure (e.g., hybrid girders, curved beams).

How do you detail a beam splice for a section with unequal flanges? For sections with unequal top and bottom flanges (asymmetric sections), design each flange splice independently for the force in that flange. The lever arm is still (d - average tf), but the tension flange force may differ from the compression flange force if the neutral axis is not at mid-depth. For large asymmetry, the web splice must also transfer the horizontal force resulting from the differential flange forces.

What minimum number of bolts is required in a beam splice per AS 4100? AS 4100 does not specify a minimum number of bolts for beam splices specifically. However, good practice is a minimum of 4 bolts per flange per side (2 rows x 2 columns) to provide rotational restraint and redundancy. For web splices, a minimum of 2 bolts per side is acceptable for lightly loaded beams. The ASI recommends that no single bolt should transfer more than 25% of the total flange force in a splice connection.

This page is for educational reference. Beam splice design per AS 4100:2020 Clause 9.5. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer registered with Engineers Australia or the relevant state registration board. Results are PRELIMINARY — NOT FOR CONSTRUCTION.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.